Differential cyclin-dependent kinase expression and activation in human colon cancer

Background: Abnormalities of the cyclin-dependent protein kinase (CDK) mach inery have been linked to cancer development. Hyperphosphorylation of the r etinoblastoma (Rb) protein results in release from inhibition of progressio n through the G1 phase of the cell cycle. Hyperexpression of CDK1 and CDK2 may enable progression through late GI, S and the G2 phases of the cell cyc le. Methods/Results. To investigate tumor-associated protein kinase activit ies, control and tumor samples were fractionated by MonoS chromatography an d tested for their ability to phosphorylate histone HI. Two major peaks of histone H1 phosphotransferase activity were resolved. The first appear ed i n the flow through? fractions, and occasionally showed enhanced activity in the tumor samples, whilst the second was consistently increased and eluted at approximately 0.4 hi NaCl. Western immunoblotting with CDK1 and PSTAIRE antibodies confirmed the co-elution of CDK1 and CDK2 with the second peak. Next, the phosphotransferase activities (following specific immunoprecipit ation) and protein levels of CDK1, 2 4 and 6 were determined in human colon cancer samples and their respective cona ols. CDK4 activity was elevated i n only 3 of 7 tumor samples (range 40-160%) relative to control samples fro m the same patients, whereas a significant increase in CDK6 activity was ob served in the same group (p<0.05). This contrasted sharply with the univers al activations of CDK1 (up to 18-fold, p<0.01, n=12) and CDK2 (up to 17-fol d, p<0.05) in the same groups. Conclusions: CDK1 especially, and to a lesse r extent CDK2 and CDK6 demonstrate the most consistent biochemical activati on in human colon cancer and may represent targets for pharmacological inte rvention. Cellular proliferation as gauged by MIBI was not directly correla ted with the amplitude of activation.